This research with hamsters will further integrate the fields of behavioral endocrinology and biological rhythms. Experiments designed for annual, circadian and ultradian rhythm analysis will benefit from the established methods practiced by researchers studying hormones of behavioral rhythms according to hormonal or lighting history; 2) tests of the external coincidence model of photoperiodism; 3) examination of an ultradian rhythm and its relationship to the neural substrate regulating a circadian rhythm.